Ammonia is a fertilizer and chemical feed stock that is typically produced in large scale plants via the catalytic reaction of hydrogen and nitrogen at high temperatures and pressures. The reaction is equilibrium limited with a hydrogen conversion of approximately 25% per pass. The reactor effluent is typically 12-18% ammonia with the remainder consisting of unreacted hydrogen, unreacted nitrogen and inerts such as methane and argon. The ammonia product is recovered from the reactor effluent in a separation system which involves repeatedly cooling and flashing the reactor effluent to effect the condensation of the "heavier" ammonia from the remaining "lights". Such cooling requires large amounts of refrigeration which makes the overall process very energy intensive. Because the amount of refrigeration which would be required to recover all the ammonia from the reactor affluent is prohibitively large, the remaining lights typically still contain 2-6% ammonia.
In order to take advantage of the large amounts of unreacted hydrogen and nitrogen contained in the remaining lights, the majority of the remaining lights are recycled to the reactor as a recycle stream. To prevent the build-up of the inerts in the process, however, a portion of the remaining lights must be purged from the process as a purge stream.
The present invention is particularly concerned with the processing of the above noted recycle and purge streams with corresponding ammonia-selective membranes. The use of an ammonia-selective membrane to recover ammonia from the recycle stream in an ammonia plant is taught in the art. See for example U.S. Pat. No. 4,758,250 by Laciak et al. Similarly, the use of an ammonia-selective membrane to recover ammonia from the purge stream in an ammonia plant is also taught in the art. See for example U.S. Pat. No. 4,793,829 by Pan et al. The present invention, however, teaches a specific and novel integration of an ammonia-selective membrane into both the recycle loop and purge loop of an ammonia plant that will realize a number of process and operational benefits.